MUSE imaging spectroscopy of the fullerene planetary nebula Tc 1

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Abstract
Aims . The planetary nebula Tc 1 (PN G345.2–08.8), one of the rare group of Galactic PNe showing fullerene emission in the infrared, is spatially extended with high a surface brightness. Optical imaging spectroscopy enables the line and continuum structure and diagnostics to be examined, and permits a search for any correlations to the presence of fullerene dust. Methods . Tc 1 was observed with MUSE wide field mode with adaptive optics, wavelength range 4700–9300 Å, at three exposure levels to ensure unsaturated emission lines. Extinction, electron temperature ( T e ), and density ( N e ) diagnostic images are presented from collisionally excited and recombination line ratios. Results . The nebula has a high surface brightness core, with a diameter 12″, an elliptical ring of major axis 2.8″ around the central star and some low ionization knots, and an extended halo of 55″ in diameter; between the core and halo is an annulus with intermediate properties, including a higher T e and a lower N e than in the core. The spectrum of the central star was extracted and fitted by a 31 000 K model atmosphere and is of type O7.5I(f). The image of optical extinction from H Balmer line ratios is highly structured, and shows an annulus, which is adjacent to the core, of low extinction, with values lower than the line-of-sight interstellar extinction. Instrumental effects to account for this anomalously low extinction area were investigated and intrinsic effects from the scattering properties of the nebular dust; neither can entirely explain the low-extinction region and the most likely cause is a local non-standard dust reddening law. This low extinction region additionally shows an anomalously high He I 7281/6678 Å line ratio, possibly caused by a contaminating line, but none have been conclusively identified; its origin remains unresolved. Spectra over extended regions were also analysed and distinct enhancement of the continuum above the expected nebular continuum (as also seen in some other PNe observed with MUSE) was found. Conclusions . The annulus of low extinction occurs outside the region of strongest fullerene emission, in the zone where N e declines and T e rises. A change in dust properties linked to conditions in this transition region between the higher density core nebula and lower density halo is deduced.

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